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Chapter 7 Photosynthesis: Using Light to Make Food PowerPoint® Lectures for Campbell Essential Biology, Fourth Edition – Eric Simon, Jane Reece, and Jean Dickey Campbell Essential Biology with Physiology, Third Edition – Eric Simon, Jane Reece, and Jean Dickey Lectures by Chris C. Romero, updated by Edward J. Zalisko © 2010 Pearson Education, Inc. Biology and Society: Green Energy • Wood has historically been the main fuel used to: – Cook – Warm homes – Provide light at night © 2010 Pearson Education, Inc. • Industrialized societies replaced wood with fossil fuels. • To limit the damaging effects of fossil fuels, researchers are investigating the use of biomass (living material) as efficient and renewable energy sources. © 2010 Pearson Education, Inc. • Fast-growing trees, such as willows: – Can be cut every three years – Do not need to be replanted – Are a renewable energy source – Produce fewer sulfur compounds – Reduce erosion – Provide habitat for wildlife © 2010 Pearson Education, Inc. THE BASICS OF PHOTOSYNTHESIS • Photosynthesis: – Is used by plants, some protists, and some bacteria – Transforms light energy into chemical energy – Uses carbon dioxide and water as starting materials • The chemical energy produced via photosynthesis is stored in the bonds of sugar molecules. © 2010 Pearson Education, Inc. • Organisms that use photosynthesis are: – Photosynthetic autotrophs – The producers for most ecosystems © 2010 Pearson Education, Inc. PHOTOSYNTHETIC AUTOTROPHS Photosynthetic Protists (aquatic) Photosynthetic Bacteria (aquatic) LM Plants (mostly on land) Forest plants Kelp, a large alga Micrograph of cyanobacteria Figure 7.1 Chloroplasts: Sites of Photosynthesis • Chloroplasts are: – The site of photosynthesis – Found mostly in the interior cells of leaves © 2010 Pearson Education, Inc. • Inside chloroplasts are membranous sacs called thylakoids, which are suspended in a thick fluid, called stroma. • Thylakoids are concentrated in stacks called grana. • The green color of chloroplasts is from chlorophyll, a lightabsorbing pigment. © 2010 Pearson Education, Inc. • Stomata are tiny pores in leaves where carbon dioxide enters and oxygen exits. © 2010 Pearson Education, Inc. Inner membrane Outer Chloroplast membrane Vein Granum Stroma Thylakoid O2 Leaf cross section Interior cell TEM Stomata LM CO2 Figure 7.2-2 The Overall Equation for Photosynthesis • In the overall equation for photosynthesis, notice that: – The reactants of photosynthesis are the waste products of cellular respiration. © 2010 Pearson Education, Inc. Light energy 6 CO2 6 H2O Carbon dioxide Water C6H12O6 Photosynthesis Glucose 6 O2 Oxygen gas Figure 7.UN1 • In photosynthesis: – Sunlight provides the energy – Electrons are boosted “uphill” and added to carbon dioxide – Sugar is produced © 2010 Pearson Education, Inc. • During photosynthesis, water is split into: – Hydrogen – Oxygen • Hydrogen is transferred along with electrons and added to carbon dioxide to produce sugar. • Oxygen escapes through stomata into the atmosphere. © 2010 Pearson Education, Inc. A Photosynthesis Road Map • Photosynthesis occurs in two stages: – The light reactions convert solar energy to chemical energy – The Calvin cycle uses the products of the light reactions to make sugar from carbon dioxide © 2010 Pearson Education, Inc. CO2 H2O Light Chloroplast NADP+ ADP P Calvin cycle Light reactions ATP NADPH O2 Sugar (C6H12O6) Figure 7.3-2 THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY • Chloroplasts: – Are chemical factories powered by the sun – Convert solar energy into chemical energy © 2010 Pearson Education, Inc. The Nature of Sunlight • Sunlight is a type of energy called radiation, or electromagnetic energy. • The full range of radiation is called the electromagnetic spectrum. © 2010 Pearson Education, Inc. Increasing wavelength 10–5 nm 10–3 nm 1 nm 103 nm 106 nm Gamma rays X-rays UV 1m MicroInfrared waves 103 m Radio waves Visible light 380 400 500 Wavelength (nm) 600 700 750 Wavelength = 580 nm Figure 7.4 The Process of Science: What Colors of Light Drive Photosynthesis? • Observation: In 1883, German biologist Theodor Engelmann saw that certain bacteria tend to cluster in areas with higher oxygen concentrations. • Question: Could this information determine which wavelengths of light work best for photosynthesis? • Hypothesis: Oxygen-seeking bacteria will congregate near regions of algae performing the most photosynthesis. © 2010 Pearson Education, Inc. • Experiment: Engelmann: – Laid a string of freshwater algal cells in a drop of water on a microscope slide – Added oxygen-sensitive bacteria to the drop – Used a prism to create a spectrum of light shining on the slide © 2010 Pearson Education, Inc. • Results: Bacteria: – Mostly congregated around algae exposed to red-orange and blue-violet light – Rarely moved to areas of green light • Conclusion: Chloroplasts absorb light mainly in the blue-violet and red-orange part of the spectrum. © 2010 Pearson Education, Inc. Light Prism Number of bacteria Microscope slide Bacterium Algal cells 400 500 600 Wavelength of light (nm) 700 Figure 7.5 Light Reflected light Chloroplast Absorbed light Transmitted light Figure 7.6 Light Reflected light Chloroplast Absorbed light Transmitted light Figure 7.6a Chloroplast Pigments • Chloroplasts contain several pigments: – Chlorophyll a: – Absorbs mostly blue-violet and red light – Participates directly in the light reactions – Chlorophyll b: – Absorbs mostly blue and orange light – Participates indirectly in the light reactions © 2010 Pearson Education, Inc. – Carotenoids: – Absorb mainly blue-green light – Participate indirectly in the light reactions – Absorb and dissipate excessive light energy that might damage chlorophyll – The spectacular colors of fall foliage are due partly to the yellow-orange light reflected from carotenoids. © 2010 Pearson Education, Inc. Figure 7.7 How Photosystems Harvest Light Energy • Light behaves as photons, discrete packets of energy. © 2010 Pearson Education, Inc. • Chlorophyll molecules absorb photons. – Electrons in the pigment gain energy. – As the electrons fall back to their ground state, energy is released as heat or light. © 2010 Pearson Education, Inc. Excited state e– Light Heat Light (fluorescence) Photon Chlorophyll molecule Ground state (a) Absorption of a photon Figure 7.8a • A photosystem is a group of chlorophyll and other molecules that function as a light-gathering antenna. © 2010 Pearson Education, Inc. Chloroplast Pigment molecules Primary electron acceptor Photon Electron Reactiontransfer Reaction center center chlorophyll a Antenna pigment molecules Transfer of energy Thylakoid membrane Photosystem Figure 7.9-3 How the Light Reactions Generate ATP and NADPH • Two types of photosystems cooperate in the light reactions: – The water-splitting photosystem – The NADPH-producing photosystem © 2010 Pearson Education, Inc. Primary electron acceptor Primary electron acceptor Energy ATP to make 2e– NADP 2e– NADPH 2e– Light Light Reactioncenter chlorophyll Reactioncenter chlorophyll H2O NADPH-producing photosystem 2e – 2 H+ + 1 2 O2 Water-splitting photosystem Figure 7.10-3 • The light reactions are located in the thylakoid membrane. © 2010 Pearson Education, Inc. • An electron transport chain: – Connects the two photosystems – Releases energy that the chloroplast uses to make ATP © 2010 Pearson Education, Inc. To Calvin cycle Light Light H+ NADPH ATP NADP H ADP P Stroma Thylakoid membrane Photosystem Electron transport chain ATP synthase Photosystem Inside thylakoid Electron flow 2e – H2O H 1 O 2 2 H H H H+ Figure 7.11 To Calvin cycle Light Light H NADPH ATP NADP H ADP P Stroma Thylakoid membrane Photosystem Electron transport chain ATP synthase Photosystem Inside thylakoid Electron flow 2e– H2O H 1 O 2 2 H H H H Figure 7.11a e– ATP e– e– NADPH e– e– e– e– Water-splitting photosystem NADPH-producing photosystem Figure 7.12 THE CALVIN CYCLE: MAKING SUGAR FROM CARBON DIOXIDE • The Calvin cycle: – Functions like a sugar factory within the stroma of a chloroplast – Regenerates the starting material with each turn © 2010 Pearson Education, Inc. CO2 (from air) P RuBP sugar Three-carbon molecule P P ADP P ATP ADP P Calvin cycle NADPH ATP NADP G3P sugar G3P sugar P P G3P sugar P Glucose (and other organic compounds) Figure 7.13-4 Evolution Connection: Solar-Driven Evolution • C3 plants: – Use CO2 directly from the air – Are very common and widely distributed © 2010 Pearson Education, Inc. • C4 plants: – Close their stomata to save water during hot and dry weather – Can still carry out photosynthesis • CAM plants: – Are adapted to very dry climates – Open their stomata only at night to conserve water © 2010 Pearson Education, Inc. ALTERNATIVE PHOTOSYNTHETIC PATHWAYS C4 Pathway (example: sugarcane) Cell type 1 CO2 Four-carbon compound Cell type 2 CAM Pathway (example: pineapple) CO2 Night Four-carbon compound CO2 CO2 Calvin cycle Calvin cycle Sugar C4 plant Day Sugar CAM plant Figure 7.14